I am going to state as simply as I can what I think is the most reasonable understanding of tznius.
The pasuk says “What does the Hashem seek of you, but to act justly and to love mercy, and ???? ??? ?? ????? – to walk quietly/hiddenly with your God? (Micha 6:8)
This is not a pasuk directed at women. It is a pasuk directed at all people. This is also not a pasuk talking about clothing. It is a pasuk talking about all aspects of life.
The idea this pasuk is conveying is that what Hashem wants is for a person to have a rich inner life; not to be focused on exteriors. That one’s avodas Hashem should be a private experience. That one’s life shouldn’t be for show.
One of many things which represent a life focused on exteriors is for a woman to dress in public in a way that is provocative to most normal men. Doing so is therefore a lack of tznius.
Tznius, as it pertains to skirt and sleeve lengths, is nothing more than this. Therefore, to find out what fits the parameters of tznius, look around the society you live in and see what will get you to be ogled. If you won’t be ogled by your average guy on the street then it’s tzniusdig. If you will, it isn’t, because knowingly causing people to ogle by definition represents a lifestyle focused on exteriors and on other people’s opinions; the opposite of a rich inner life.
Most importantly, the bottom line is that there is NO absolute rule. It all depends on how society perceives things.
Looking at all the Gemaras I have seen I do not notice anything which contradicts this understanding. On the contrary, I think it is the most reasonable.